The present invention relates to the field of transceivers, and, more particularly, to a receiver section of a telephone.
A signal received by a telephone from a digital telephone network is demodulated, processed in digital form, converted to analog form, and is amplified before being applied to an electroacoustic transducer. The receiver section of a telephone can be represented schematically as shown in FIG. 1.
A demodulated digital signal RX-IN produced by a receiving and demodulating unit 10 is applied to the input of a processing unit 11. The processing unit 11 converts the demodulated digital signal to analog form, filters and amplifies the analog signal. The analog signal leaving the processing unit 11 is applied to the input of a power amplifier 12 for transferring the signal to an electroacoustic transducer 13, such as an electromagnetic earphone or speaker, with sufficient electrical power for its operation.
The gain of the power amplifier 12 is determined by the ratio between the resistance of a feedback resistor R2 connected between the output and the inverting input of the amplifier, and by the resistance of a resistor R1 connected between the output of the processing unit 11 and the inverting input of the amplifier.
To prevent the dissipation of power when the telephone is not in use, circuit means are provided both in the processing unit 11 and in the amplifier 12 to respond to an external enabling or disabling command. This function is initiated via a terminal connection of the processing unit 11 and via a terminal connection of the amplifier 12 in response to a digital power down PD signal. The power down PD signal may be a single bit signal. The receiver section is put into a state of zero current absorption when PD=1 (power down), and is enabled to absorb power supply current when PD=0 (power up).
When a change is made from the power down state to the power up state, or vice versa, both the processing unit 11 and the power amplifier 12 are subjected to abrupt voltage and current transients before returning to a normal operating condition. During these transients, electrical disturbances are produced by components having frequencies in the acoustic band, which results in an audible noise in the speaker 13 that may be annoying.
Various arrangements requiring the addition of active or passive components in series and/or in parallel with the speaker 13 for filtering undesired electrical disturbances have been adopted to overcome this problem. This approach, however, requires relatively bulky components outside the power amplifier which may normally be formed as an integrated circuit.
An object of the present invention is to provide a receiving section for a telephone which is not subject to the problem described above and which can easily be integrated.
This and other objects of the present invention are provided by a receiver section of a telephone comprising a demodulator unit for receiving and demodulating an input signal, and a processor unit for processing the demodulated input signal. The processor unit includes balanced outputs. An amplifier unit including balanced inputs is coupled to the balanced outputs of the processor unit, and the amplifier unit includes balanced outputs coupled to a speaker.
A switch circuit is coupled between the balanced outputs of the processor unit and the balanced inputs of the amplifier unit. A control unit generates a closing control signal for closing the switch circuit according to a predetermined delay with respect to a power up signal for preventing disturbances in the processor unit due to transients of the power up signal from appearing on a processed input signal being applied to the amplifier unit.